Abstract:

XRD and DTA were utilized to investigate quantitatively the clay-mineral composition of the argillaceous rocks in four Miocene stratigraphic sequences located in western and southwestern Qatar. These rocks belong to the Dam Formation and are more concentrated in its lower Member "A" than in the overlying Member "B" which is dominated by a carbonate-evaporite fades. The reported clay-mineral suite consists of (in order of decreasing abundance): illite, kaolinite, attapulgite and Ca, Mg-montmorillonite. Illite and kaolinite are intimately associated and their crystallinity characters at a given stratigraphic horizon are closely similar. Well-crystallized attapulgite is present mainly in the marlstones especially those associated with evaporites. The occurrence of montmorillonite is restricted to the siltstones occupying the base of Member "A". Generally, Member "B" sequence consists of three thick illite-attapulgite-kaolinite zones alternating with two thin illite-kaolinite zones.
Evidently, illite and kaolinite were formed by detrital inheritance from weathering horizons and/or soils on the Precambrian rocks in western Saudi Arabia. Montmorillonite seems to have been neoformed in the basin of deposition under conditions of alkalinity and high AP'^/Mg2"*" ratios. It is also probable that the mineral was formed diagenetically through hydration of illite by pore solutions having high H+/K+ ratios. Attapulgite had also originated in the basin of deposition mostly by alteration of montmorillonite and/or magnesium-rich debris derived from weathered basic igneous rocks.